1. Field of the Invention
The present invention relates to a method of preparing a lithium metal anode, and more specifically, to a method of preparing a lithium metal anode with high energy density.
2. Discussion of the Related Art
With the increasing need for smaller portable electronic devices, such as camcorders, communication devices, and computers, the need for more compact, lighter, thinner and capable batteries to drive such devices has also increased. Conventional lithium ion secondary batteries may utilize a transition metal oxide (such as LiCo O2) as a cathode active material and carbon as an anode active material. Carbon has a theoretical capacity of 372 mA/g, resulting in a battery with low energy density.
However, when using lithium metal as an anode instead of carbon, a battery has higher energy density and lower weight since lithium has the highest potential difference (−3.045 V vs a standard hydrogen electrode (SHE)) among metals and a low density (0.53 g/cm2), to go along with its high theoretical capacity of 3860 mAh/g. But due to its high reactivity, lithium metal tends to react with oxygen, nitrogen, and carbon dioxide when exposed to air, even at room temperature, thereby forming Li2CO3, Li2O, LiOH and other like substances on its surface. Thus, it is very difficult to obtain lithium metal with a clean surface. Additionally, lithium metal tends to react with an electrolytic solution to form a passivity layer, which is not chemically or physically uniform, thereby causing a localized current density on an electrode surface. This facilitates localized dendrite growth, resulting in a short circuit in the battery. Moreover, the formation of dead lithium on the anode reduces its capacity. Accordingly, an organic, inorganic or organic/inorganic hybrid thin film may be formed on the lithium metal surface to inhibit a reaction between the lithium metal and the electrolytic solution. However, if the lithium metal surface to be coated is not initially clean, the thin film does not function well as a protective layer.
To overcome these problems, a method has been developed in which a lithium metal is vacuum deposited on a substrate to form a lithium layer with a clean surface, and an organic, inorganic or organic/inorganic thin film is formed thereon before exposing the product to air. In this method, the substrate may be composed of a metal, including copper, aluminium or nickel, or a film of polymer, including polyester, polyethylene, polypropylene or polyimide. However, depositing the lithium metal generates a lot of heat, which will likely deform a thin substrate. Thus, it is necessary to use a substrate that is at least about 15 μm thick. However, a 15 μm thick substrate may result in a battery with low energy density.
U.S. Pat. No. 6,214,061 discloses a method of preparing an electrode in which a 50 Å copper film is formed as a release layer on a substrate composed of polymer. An inorganic film, a lithium film and a current collector are sequentially formed on the release layer, and the polymer substrate is then released. However, a release component of copper film has poor release properties, and after the substrate is released, copper may remain on the protective inorganic layer, thus inhibiting lithium ion movement and deteriorating the battery's performance.